This invention relates to a system for providing reset action in a controller and, more particularly, the use of an averaging circuit to provide a simple means to produce reset action.
Automatic proportional control is used in many processes, for example temperature control within an oven. Typically, a temperature sensor within the oven produces an output signal, dependent upon the temperature within the oven, which is compared by a comparator to a signal indicative of the desired temperature within the oven, i.e., the setpoint. The output from the comparator is proportional to any deviation between the actual temperature and the desired temperature. This output is typically amplified and used to control either a valve which in turn controls the amount of gas flowing to a burner located within the oven or to a power amplifier for controlling the amount of current supplied to an electric heater within the oven. If a valve is the final control element, for example, the amplifier may be designed to supply an 8 volt output when the actual temperature equals the desired temperature. The valve, at 8 volts, is maintained at its mid-position. Any deviation of the actual temperature away from setpoint results in a change in the output voltage of the amplifier to move the valve away from its midposition. Alternatively, the amplifier may be designed to have 0 output voltage when the actual temperature equals the desired temperature. The valve may be designed to operate at its mid-position with a 0 volt input.
Thus, any change of the actual temperature away from set point again results in a change in the output voltage of the amplifier to move the valve away from its mid-position.
If a load change should occur, such as for example from a change in conditions either external or internal of the oven, the temperature within the oven will begin to change. The output from the comparator and the amplifier will change in proportion to the change in temperature within the oven and the valve is proportionally repositioned. However, since the oven now requires more heat (or less if the deviation from set point is in the other direction) than when the system is in balance, i.e., when the actual temperature equals the set point temperature, the comparator and amplifier must deviate from their mid-point output in order to maintain the valve at a position to insure that more heat (or less) is supplied to the oven.
In order to maintain this valve at its new position to meet the new conditions, the output from the amplifier must be such as to hold the valve at this new, non-middle position. Thus, the amplifier output must be a value other than that which it had when the actual temperature equaled the desired temperature and this value can only result when the actual temperature is not equal to the desired temperature. This difference between the actual temperature and the desired temperature which is necessary to maintain the valve at its new position is called "offset" or "droop."
This action means, of course, that the temperature within the oven will be maintained, not at the desired temperature, but at the "offset" temperature. When the system is under manual control, the operator will notice a departure of the temperature within the oven from the desired temperature within the oven and will readjust the valve or power amplifier accordingly. However, when the system is in automatic control, a proportional control system is insufficient of itself to compensate for this "offset."
To overcome this deficiency in automatic control systems, the prior art turned to reset. Reset simply forces the process back to the setpoint by shifting the proportional band up or down the scale of a controller until the required valve opening or power amplifier energization returns the variable, i.e. the temperature, to the control or set point. In effect, therefore, reset action produces a continuing change in the output until the control variable is again equal to the desired set point. The present invention relates to a means for providing this reset action.